Before embarking on the main features of the invention proper, it is appropriate to recapitulate the difficulties encountered when adapting motors, eg. internal combustion engines, to hang glider type aircraft, and also the difficulties encountered at take off and during various stages of flight.
Hang gliders have already been fitted with wheeled frames constituting a kind of nacelle for supporting a person suspended from the neutral point of the wing. The user is seated in the nacelle which generally includes an internal combustion engine fixed behind the user. The motor and frame assembly ensures good "hanging" type piloting conditions.
However, there are several drawbacks associated with this type of aircraft that have limited its development. Firstly the motorized nacelle is bulky and presents transport problems which detract from the feature of easy transport associated with the non-motorized hang glider.
The weight of the nacelle is also a serious functional drawback since it requires the motor to operate continuously to keep flying. If the motor stops, the sink rate is high.
Generally speaking such nacelles are bare frames without any kind of streamlining, so another functional drawback is the high aerodynamic drag of the nacelle together with the seated pilot and the motor. To mitigate these drawbacks, it is tempting to increase both the wing area and the motor power of the aircraft. Under such conditions, the motorized hang glider ceases to be a very light and maneuverable aircraft suitable for gliding sport.
Other means have also been devised to motorized a hang glider without resorting to a wheeled nacelle. For example the motor propeller unit may be fixed under the keel of the wing in one of two positions relative to the center of gravity: a first position in which the motor is behind the center of gravity with the propeller in front, thus constituting a puller system; and a second position in which the motor is in front of the center of gravity with the propeller behind, thus constituting a pusher system.
These types of solution give rise to weight and drag penalties which are acceptable in comparison with the hanging nacelle: the pilot can now be in a prone position hanging from the wing by means of a harness.
Unfortunately, mounting the power unit on the keel of the wing gives rise to piloting difficulties which may be dangerous since by definition a hang glider is controlled in flight by the pilot moving bodily relative to the wing's center of lift. The weight of the fixed motor reduces the effect of the pilot's movements and makes control of the aircraft in flight a chancy undertaking. The acrobatic capacity of the wing is thus likewise considerably reduced by the presence of the fixed motor.
A third system consists in strapping the motor to the pilot's back. In this case the combined pilot plus motor weight increases the effects of pilot body movement on aircraft control, but adds very considerably to the danger in the event of a crash. Also the drag is very considerably increased due to the fact that the pilot must remain in an upright position during flight. Another difficulty stems from the fact that take off is possible only from a suitable slope.
Indeed motorized take off is also difficult with a fixed propulsion unit, except in the case of the wheeled nacelle or cart, but in that case the drag and the weight of the nacelle considerably restrict the acrobatic performance of the motorized hang glider once airborne.
Preferred embodiments of the present invention mitigate the various drawbacks mentioned above by providing a propulsion unit which is adaptable to a hang glider type of aircraft and which enables it to be piloted in safety without modifying the initial flying characteristics of the hang glider.
Additionally, such preferred embodiments of the invention are not carried by the pilot, are easy to use at take off, even from flat ground without a near by slope, are capable of taking the aircraft up to a desired height; and may then be turned off for genuine gliding flight with minimum pilot drag.